Greenhouse gas flux in canopy soils and forest floor soils in coastal old-growth bigleaf maples in temperate rainforests of Western Washington

Abstract

As a whole, forest ecosystems sequester carbon from the atmosphere and serve as an enormous pool for carbon storage in their soils and vegetation. As climate change accelerates due to the buildup of greenhouse gases (GHGs) in our atmosphere, it is increasingly important to understand how these carbon pools exchange GHGs with the atmosphere and how these gas fluxes are influenced by environmental change. The Hoh Rainforest in the Olympic Peninsula of Washington state is well recognized for its large quantities of epiphytes and canopy soil on tree branches, which can serve as an additional pool of carbon and site for greenhouse gas flux. This study is a first attempt to measure greenhouse gas flux from Hoh Rainforest canopy soils and quantify their contribution to the overall flux by these forest soils. Data collection to develop the sampling protocol showed that emissions of carbon dioxide and methane from canopy soils were not significantly related to distance from the bole or branch height. Here we show that temperate rainforest canopy soils are a source for carbon dioxide emissions and, on average, a sink for atmospheric methane, though flux rates for both gases were slower in canopy soils than those measured on the forest floor soils. After scaling for the surface area of each soil type found in these ecosystems, canopy soils contributed about 4% to overall soil carbon dioxide emissions and 4% to overall soil methane sequestration. While carbon dioxide emission per unit surface area in these canopy soils was comparable to measurements in tropical forest canopy soils, methane sequestration was greater than that seen in other studies. Moisture and temperature did not have a clear influence on GHG flux rates in this study, emphasizing the need for more sampling to understand potential effects of environmental change on these canopy soil systems. By measuring greenhouse gas flux in these smaller carbon pools, we can better understand the nuances of carbon cycling in these forests and how these unique ecosystems may respond to a changing climate.